1. Field of the Invention
The subject invention relates generally to digital frequency synthesizer apparatus and, more particularly, to apparatus for correcting errors in such apparatus resulting from the use of a limited number of digital bits to represent an analog signal.
2. Description of Related Art
The Direct Digital Synthesizer (DDS) provides a means for achieving very fast frequency hopping in radio frequency transmitters while retaining phase coherence between transmitter and receiver. However, for several reasons, present DDS technology does not produce sinusoidal output accuracy sufficient for HF-radio requirements.
Conventional DDS circuitry employs a sine table look-up operation to output a series of digital sine approximation values and a digital-to-analog converter (DAC) to convert the digital sine approximation values to an analog output waveform. The prior art uses as many bits in the look-up table as can be accommodated by the DAC. Truncation errors result from memory limitations, including limitations on the number of memory addresses and the number of bits (size) of the memory output word. The size of presently-available DACs also contributes to error in the conventional DDS output. As a result of such errors, the present state of the art provides accuracy on the order of -60 dBc spurious (about 10-bit actual accuracy) with a 12-bit DAC output, whereas -80 dBc is required for HF radio applications. To achieve -80 dBc spurious output, resolution of 14-bit accuracy is required. However, DACs having 14-bit output resolution are presently unavailable.